You can't UNDO, but if anyone else posts an Answer that you feel solves the problems, then you can change your "Best Answer" to the new one.
I wouldn't worry about it. You have given more information in your existing thread so wait for more responses.
Hi. I asked a question and someone answered me. I clicked "<span id="INSERTION_MARKER"> This post answered my question" button by fault. I wanted to undo it, cause I couldn't do this. I asked my question again. and my question was blocked. How can I undo " the problem is resolved"?